Frequency comparison apparatus



Nov. 21, 1945. A, H BARTELS 2,389,948

FREQUENCY COMPARI-SON APPARATUS l Filed April so. 194:5 2 sheets-sheet 1 ARNOLD H. BARTELS ATTORNEY Nov. 27, 1945. A. HfBARTELs A 2,339,948

FREQENCY .COMPARISON APPARATUS Filed Apri1 30, 1945 2 Sheets-Sheet 2 FIG,.2 48

INVENTOR ARNOLD H.BARTELs 1 l l BY ...4A 'N'W ATTORNEY Patented Nov. 27, 1 945 v i i 2,889,948 FREQUENCY COMPARISON APPARATUS Arnold H. Bartels, Fort Wayne, Ind.,v assigner to Farnsworth Television and Radio Corporation, a corporation of Delaware Y Application April 80. i948, seriaruaisazos (ci. 25o-sel 13 Claims.

'I'his invention relates to frequency control apparatus and particularly to apparatus for con trolling the frequency of an oscillator used in a, television timer.

According to conventional television practice the synchronizing signal generator Aand the transmitter scanning generators are operated in a definite time relation to a source of standardizing frequency which usually is a commercial alternating current power supply. This relationship is attained by the use of so-called timer ap-f paratus which includes a high frequency oscil- .lator and a timer chain or frequency divider,

whereby the scanning and synchronizing signal frequencies are produced. A submultiple frequency of' thehigh oscillator frequency is compared with the alternating current power supply frequency to generate a control voltage representative of `the frequency comparison. This voltage is employed to control the operation of` an automatic frequency control circuit, whereby the oscillator frequency may be locked in a harmonic relation with the source of. standardizing frequency. A

As is well understood in the art, the control voltage generally is in the form of a series4 of spaced impulses. In order to utilize these impulses they are passed through a filtering net work to produce a voltage which approximates a steady direct voltage. 1t is necessary to provide filtering for the control impulses in order `to prevent the frequency of the oscillator from Another object of the invention is to provide,

in automatic frequency control apparatus for a television timer, means whereby to minimize the ltering necessary to convert a series ofcontrol impulses into an approximation of a steady direct current control voltage.

A further object of the invention is to provide, in` automatic frequency control apparatus for a television timer, means whereby to maintain -frequency of the first of the two sources.

of the frequency of one ofthe sources.

synchronism between an oscillator and a source of standardizing frequency, irrespective of sub.

stantial uctuations lof the voltage lof the standardizing source.

VIn accordance with the present invention, there is provided a novel circuit for producing a voltage representative of the phase relation between impulses derived from. two periodic sources. This circuit includes a means for deriving from one of the sources a series of impulses having a frequency which is a harmonic of the` frequency of the source from which it is derived. Additional means is provided for deriving from the other of the two sources, a series of impulses also having a frequency which is a harmonic of rlle e frequencies of the two series of impulses are preferably but not necessarily the same harmonic There also is provided, means for combining the two series of impulses in a manner to produce a composite wave which has pedestal portions representative of the phase relation between the two series of impulses. The novel circuit also includes a means for developing from the composite wave substantially continuous unidirectional voltages which have magnitudes corresponding to the energy content of the pedestal portions.

More specifically, in accordance with the i1- lustrated embodiment of the invention, there is provided an automatic frequency control system having means for converting a standardizing alternating current into a series of steep sided impulses having sloping crests at a harmonic frequency of the alternating current. 'I'here also is provided, means for deriving from the television timer oscillator a series of substantially saw-toothed impulses. These two series of im,- pulses are combined to form a composite Wave having pedestal portions. The peak amplitudes and the widths of the pedestal portions are rep; resentative of the phase relation between the two series of impulses. There also is provided an integrating means, whereby energy derived ,from the pedestal portions is utilized to produce a control voltage which approximates a steady direct voltage. The control voltage is applied to an automatic frequency control circuit, whereby to effect a change in the frequency of the oscillator derived impulses in accordance with changes in the control voltage.

By reason of the wave shapes of the two series of impulses a slight phase change of one series with respect to the other is manifested by a change in the peak amplitudes of the composite wave pedestals and also by a pronounced change in the widths thereof. Consequently, the manner of combining the two series of impulses to form the composite wave provides a means for generating impulsive energy which represents very slight frequency deviations of one series of impulses with respect to the other. Also, since the energy impulses are generated at a rate which is a multiple of the stadardizing frequency the integrating network is provided with smallerv storage facilities than have been possible heretofore. The control voltages generated as a result are much more responsive to frequency changes and thus, the frequency stability of the oscillator is considerably improved by virtue of the fact that a differential control voltage is applied oftener in a given time interval than was possible in prior art systems.

For a better understanding of the invention together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings and its scope will be pointed out in the appended In the accompanying drawings:

Fig. l is a partially schematic circuit diagram of automatic frequency control apparatus .embodying the invention; and

Fig. 2 is a series of graphical representations of wave forms of the voltages appearing in various circuits of the apparatus which are of as- 1- sistance in explaining the operation of the appaoscillator II which may be any conventional type of oscillator.' The oscillator is designed to generate oscillations at a relatively high' frequency. These oscillations are impressed upon a conventional timer chain or frequency divider I5 whereby to subdivide the oscillator frequency so as to produce impulses at the desired synchronizing and scanning frequencies. The components of the frequency divider may also be conventional blocking oscillators, multivibrators or.the'like. There also is derived from the frequency divider I5, impulses of a. predetermined frequency which are employed in a manner to be described subsequently to assist in the generation of an automatic frequency control voltage.`

The relatively. low frequency impulses derived from the last stage of the frequency divider I5 arecoupled bymeansofacondenser Itothe control grid of an amplifier tube which is illustrated as a triode I1. The cathode of the tube I1is connected through a biasing resistor I3 toothed voltage waves which appear at the anode of the tube.

Such impulses are utilized foi` the purpose of comparing the frequency thereof with a source ofstandardizing frequency. Such a sourcemay be a commercial alternating current power supply which is connected to the primary winding of a transformer 23. The secondary winding of the transformer is shunted by a series arrangement of two resistors 24 and 25. These resistors are for the purpose of placing the center of the winding at ground potential and may be omitted, if desired, where the winding itself may be center tapped. The terminals of the secondary winding are also connected to the anodes of a twin diode rectifier tube 26. The cathode of this tube is connected through a load resistor 21 to the grounded junction point of resistors 24 and 25. The rectifier tube and its associated circuits functions as a frequency multiplier. While obviously not limited to any particular multiplication of the power supply frequency, as disclosed herein for illustrative purposes only. the apparatus serves to double the power supply frequency.

The double frequency voltage appearing across the resistor 21 is coupled by means 'of a condenser 28 to the control grid of an amplifier tube illustrated as a triode 29. The cathode of this tube is connected through a self-biasing resistor 3l to ground, and to the grounded terminal of the rectifier load resistor 21. A leak resistor 32 is provided for the control grid of the tube 29.

Space current for the tube 29 is supplied from a source of direct current energy such as the battery 22, the positive terminal of which is connected to the anode of the tube through a resistor 33. The magnitude of the voltage applied to the control electrode and the biasing of the input circuit of the tube 29 is of such a nature that the tube. operates in the anode-to-cathode current saturation region during a portion of the input cycle. As a result, the wave form of the voltage appearing at the anode of the tube 29 is substantially flat topped and the slopes o the sides of the wave are relatively steep. A

T he voltage developed at the anod'e of the tube 2l is coupled by means of a relatively small condenser 34 to the control grid of another amplitoground. There also is provided a leak resistor l I! for the control grid of the tube I1. The coupling circuit including the condenser Ii and the resistor il is designed to have a relatively small time constant for a purpose to be described. As dclosed herein, this result is achieved by the use of a relatively small condenser, but, obviously.

the use of a relatively small resistor will accom- Blish the same result. The anode of the tube I 1 is connected by a. conductor 2l and a resistor 2I to the positive terminal of a sourceof direct current energy auch as a battery 22. The negative terminalotthisbatteryisconnecteddirectlyto ground- Becameottherelatlvelylowcapadty of the couplhg condenser I6 the impulses derived Vfrom thevfrequaxcy divider I 5 are converted in the output circuit ofthe tube I1 into -sawiler tube which is illustrated as a triode 35. The cathode of this tube also is connected through a biasing resistor 36 to ground. A leak resistor 31 is provided for the control grid of the tube 35.

.This coupling circuit is designed to have a relatively small time constant which, alternatively, may be attained by the use of a relatively small resistor 31, in the manner indicated in connection with the description of the coupling circuit for the tube I1.

The anode ofthe tube 35 is connected through resistors 38 and 2| tothe positive terminal of the battery 22. The tube 35 also is biased for operation inthe anode-to-cathode current saturation region during a portion of the input cycle.

Consequently, the wave form of the voltage developed at the anode of the tube 35 has leading and trailing edges which are substantially perpendicular. Also,- the tops or crests of the voltage wave are sloping. The sloping crests of the'voltage wave are produced by the relatively small capacity of the coupling condenser 34 so ply and the substantially saw-toothed wave derived from the frequency divider l during normal operation of the apparatus are identical. It is contemplated to be within the scope of this invention that these frequencies may be any desired harmonically relatedfrequencies. The composite wave producedby the combination of the two series of impulses which appears at the Junction point of resistors 38 and 2| has pro nounced pedestal portions recurring at the frequency of the impulses.

The composite wave is applied to an integrating circuit-whereby to convert the impulsive energy into substantially direct current energy. The integrating circuit comprises a series arrangement of a condenser 39 and a resistor 4I. This series arrangement is connected to a series parallel arrangement of resistors 42 and 43- and the condenser Il. The energy of the impulses of the composite wave form is converted into a unidirectional voltage which appears across the terminatingcondenser M of the integrating network.

The unidirectional control voltage is applied through a resistor 45 to the input terminals of an automatic frequency control circuit 46. The output terminals of the A.` F. C. circuit are connected to the frequency determining circuit of the oscillator Il., whereby to control the frequency thereof in accordance with the control `voltage developed across the condenser All.

Referring to Fig. 2, typical wave forms of the voltages appearing in different parts of the circuit oi' Fig. 1 are shown. The curve I1 illustrates n approximately the wave form of the voltage appearing at the anode of the tube 29. It is noted that, because of the operation of the tube in the anode-to-cathode current saturation region dur--v ing a portion of each cycle, the voltage impulses have substantially fiat crest portions 4t. Also, the leading and trailing edges 49 and 5d respectively, of each impulse are relatively steep. This is also the result of the operation of the tube ld in the saturation region'.

.e voltage having a wave form Aauch as the curve 4l is converted, by the tube 35 and its exillustrated by the curve 5I and the voltage having the wave form such as illustrated by the curve 5B are combined m the interconnected output circuits of the tubes 35 and l1. The resulting composite voltage which appears at the .lunction point between resistors 38 and 2| has a wave form similar to that illustrated by the curve El. It is seen that each impulse of the combined voltage wave has a pedestal portion 59. The pedestal portion is produced by the super-position of the peak portion of the saw tooth. wave 55 upon the sloping crest portion 54 of the voltage wave El. A

Referring now to the operation of the apparatus illustrated iii-Fig. l, assume that the frequencyvof the oscillator derivedimpulses is the same as that of the impulses derived from the alternating current power supply. If the power supply has the usual frequency, of 60 cycles per second, the frequency of the impulses derived v therefrom and also the frequency of the oscilternal circuit elements, into a voltage having.

substantially the form shown by the curve tl. Since this tube also is operated in the anode-tocathode current saturation region, during a portion of each cycle, the leading and trailing edges M and 53 respectively, of each impulse are substantially perpendicular. Also each impulse has a sloping crest portion'il. It is noted that the crest portion slopes somewhat downward from the `leading edge to the trailing edge of the impulse. 'The form of this portion 'of the impulse results from the employment of the coupling circuit with small time constant which includes the relatively small capacity condenser 34.

The curve 55 illustrates the wave form of the impulses applied to the input circuit of the tube l1. Each impulse, as derived from the frequency divider l5, has a steep leading edge 5B and, as impressed upon the grid of the tube l1, has a sloping trailing edge 51. The configuration of the trailing edge is produced by the employment of the coupling circuit with small time constant which includes the relatively small capacity condenser Ii as one of the coupling elements between the frequency divider Il and the tube I1. Thus, the impulses derived from the frequency divider have wave shapes which are substantially saw-toothed in form. l

' The voltage having the wave form such a8 lator derived impulses will be cycles per second. This is a result of the-operation of the full wave rectifier including the tube 26 as a frequency doubler. Under the assumed conditions the oscillatorderived impulses illustrated by the curve 55 'of Fig. 2 are related in phase to the impulse derived from the alternating current power supply ,illustrated by the curve 5l, as indicated in Fig. 2. The resulting composite voltage wave represented by the curve til is dltered by means of the described facilities to produce a normal control voltage which has a substantially constant unidirectional value. This-voltage appears at the terminals of the condenser 4t and is applied to the'input terminals of the automatic frequency control circuit d@ DY means of ,which the frequency of the oscillator il is loclsed in predetermined harmonic relation with the alternating current power supply.

Il, now, lthe frequency of the oscillator derived impulses increases slightly with respect to the frequency of the impulses derived from the alternating current power supply, the saw-toothed voltage wave 55 of Fig. 2 will lead in phase the` voltage wavebl., This condition is illustrated by .the broken line curve 6l. When such a'voltage wave is combined with the voltage wave 5l there is produced at the junction point between resistors te and 2l a composite wave having the form illustrated by the curve 62. It will be noted that .thepedestal portion 63 of the impulsive voltage is slightly increased in its peak value due to the sloping crest 5t of the voltage wave 5l. Also, the width of the-pedestal portion 63 is increased by an amount corresponding to the phase shift. When a series of' such impulses is filtered by means of the facilities provided for such purpose there is produced a voltage across the condenser 4l which is representative of the sense of the frequency deviation and the magnitude thereof.

vWhen this voltage is applied to the automatic saw-toothed wave II oi' Fis.- 2 in a sense opposite tothat described. Asaresultofsuchaphase shift the pedestal portion Il of the composite voltage wave i8 is changed correspondingly whereby to produce `a representative voltage` across the condenser Il of Fig. 1. When this voltage is applied to the automatic frequency control circuit 46, the oscillator il is corrected to restore the frequency of the impulses derived from the frequency divider l to the predetermined relation with the standardizing frequency.

It is important to note that the frequency comparison circuit in accordance with the instant invention provides a means for maintaining synchronism between the oscillator and the source of standardizing frequency even though the voltage of the latter uctuates over a rather wide range. This perfomance feature is obtained by the operation of the tubes 28 and 35 in the respective anode-tc-cathode current saturation regions. Since these tubes are operated at saturation for a substantial portion of each alternating current cycle, it is evident that the peak amplitudes of the alternating current wave may o vary appreciably without having any noticeable influence upon the output voltage of the tubes 29 and 35. As a result, substantially large variations of the alternating current power supply voltage may occur without affecting the capability of the system to maintain the desired synchronism.

While it will be understood that the circuit specications of the desired apparatus may vary according to the design for any particular application, the following circuit specifications for a frequency comparator employed for the purpose described are included by way of exampleI only:

Condenser IG 0.01 microi'arad Tube I1 M of 68N? Resistor i! 1,000 ohms Resistor i9 270,000 ohms Resistor 2l 10,000 ohms Battery 22--.' 325 volts Resistor 24' 10,000 ohms Resistor 25 10,000 ohms Tube 26 6116 Resistor 21 100,000 ohms Condenser 28 0.5 microtarad Tubes 29 and 3l 68N? Resistor 3i 1,000 ohms Resistor 32 270,000 ohms Resistor 33 10,000 ohms Condenser 34 0.03 microfarad Resistor 3l 1,000 ohms Resistor 31 270,000 ohms Resistor 3L V-- 15,000 ohms Condenser 39 0.25 microfarad Resistor Il .100,000 ohms Resistor I2 100,000 ohms Resistor 43 680,000 ohms Condenser e-; 0.1 microfarad Resistor l5 -1. 10,000 ohms Thus, there is provided by the novel phase comparator described, a means for applying a corrective voltage to the A. F. C. circuit, whereby the frequency"` determining circuit clan oscillator is maintained in a predetermined harmonic Y relation with a standardizing frequency. 'I'he Y parison at a higher frequency than usually is assaess the practice, whereby to permit a decrease in the amount of filtering necessary to` produce the necessary control voltages. Also, by reason of the descibed shaping of the impulses derived from the source of standardizing frequency. there is provided an output voltage which is substantially constant over the range of line-voltage fluctuations which are customarily encountered in practice. Moreover, as a further, result of the impulse shaping, synchronization of the apparatus is achieved in approximately thirty seconds after the power is initially applied to the apparatus.

While there has been described what, at present, is considered the preferred embodiment of the invention, it will be obvious .to those skilled in the art that various changes andy modiiications may be made therein without departing from the invention, and therefore, it is aimed in the appended claims to cover al1 such changes and modificationsi as fall within the true spirit and scope of the invention. Y

What is claimed is:

1. A circuit for producing a voltage representative of the phase relation between impulses derived from an oscillator and impulses derived from a source of standardizing frequency comprising, means for producing from said source of standardizing frequency a first series of steep sided impulses at a harmonic of the standardizing frequency, means for producing from said oscillator a second series of substantially saw-toothed impulses, each having one edge of relatively great slope and the other edge of relatively small slope, means for combining said steep sided impulses and said saw-toothed' impulses to produce a composite wave having pedestal portions, of substantially unvrying amplitudes and varying widths representative of the phase relation between said two types of impulses, and means for developing from said composite wave substantially continuous unidirectional voltages having magnitudes corresponding to the energy content o'f said pedestal portions.

2. A circuit for producing a voltage representative of the phase relationship between impulses derived from an oscillator and impulses derived from a source of standardizing frequency comprising. means to produce a rst se- 'ries of impulses at a harmonic of the standpulses having leading edges of relatively great slopes and trailing edges of relatively small slopes, means for .combining said steep sided impulses and said saw-'toothed impulses to produce a composite wave having pedestal portions of varying amplitudes and widths representative of the phase relation between said two types of impulses. and means for developing from said compositeY wave substantially continuous unidirectional voltages having magnitudes corresponding to the energy contentof said pedestal portions. v

3. A circuit .for producing a voltage represent-- Vative ofthe phase relation between impulses derived from an 'oscillator Vand impunes derived from a sourceof standardizing frequency compricing. a frequency multiplier to produce a le- `ries of impulses at a harmonic of the standardizing frequency, first electronic means coupled to said frequency multiplier for converting the impulses derived therefrom into a series of steep sided impulses, a frequency divider con-I nected to said oscillator for producing a series of impulses at a harmonicof the standardizing frequency, second electronic means coupled to said frequency divider for imparting to said oscillator derived impulses saw-toothed wave forms having leading edges of relatively great slopes and trailing edges of relatively small slopes, means for combining said two series of impulses, whereby to produce a composite wave having pedestal portions of varying amplitudes and widths representative of the phase relation between said two series of impulses, and means for developing from said composite wave substantially continuous unidirectional voltages having magnitudes corresponding to the energy content of said pedestal portions.

4. A circuit for producing a voltage representative of the phase relation between impulses derived from an oscillator and impulses derived from a source of standardizing frequencies comprising, a frequency multiplier connected to said source of standardizing frequency to produce a first series of impulses having a frequencyharmonically related to the standardizing frequency, an electron discharge device for converting the impulses derived from said frequency multiplier into a series-of steep sided impulses, a ilrst coupling circuit having alrelatively small time constant linking said multiplier with said electron discharge device for shaping said steep sidedimpulses to have sloping crests, a frequency dividerconnected to said oscillator for producing a second series of impulses having a frequency harmonically related to the standardizing frequency, a second electron discharge device associated with said frequency divider, a second coupling circuit having a relatively small time constant linking said frequency divider with said second "electron discharge device for shaping said oscillator derived impulses to have sawtoothed wave forms, means for combining said two series ofrshaped impulses, whereby to produce a composite wave having pedestal portions occurring at the frequency of said oscillator derived impulses, said pedestal-portions having energy contents representative of the phase relation between said two series of shaped impulses, and means for developing from said composite wave substantially continuous unidirectional.

voltages having magnitudes corresponding to the energy content of said pedestal portions.

5. A circuit for producing a voltage representative of the phase relation between impulses de.

rived from an oscillator and impulses derived' forms to said oscillator derived impulses, means for combining said two series of impulses, whereby to produce a composite wave having pedestal portions occurring at the frequency of said oscillator derivedimpulses, said pedestal portions having energy contents representative of the phase relation between said two series of impulses, and means for developing from said composite wave substantially continuous unidirectional voltages having magnitudes corresponding to the energy content of said pedestal portions.

6. A circuit for producing a. voltage representative of the phase relation between impulses derived from an oscillator and impulses derived from a source of standardizing frequency comprising,l a frequency doubler connected to said.

source of standardizing frequency to produce a series of impulses of twice the standardizing frequency, a vacuum tube coupled to said frequency doubler for converting the impulses derived therefrom into a series of steep sided impulses, a relatively small capacity coupling condenser associated with said tube for imparting sloping crests to said steep sided impulses, a frequency divider connected to said oscillator for producing a series of impulses having twice the standardizing frequency, a second vacuum tube' coupled to said frequency divider, a relatively small coupling condenser associated with said second vacuum tube for imparting saw-toothed. wave forms to said oscillator derlved impulses, a common output circuit for said vacuum tubes for combining said two series of impulses, whereby to produce, a composite wave having pedestal portions, said pedestal portions having energy contents representative of the phase relation between said two series of impulses, and means for developing from said composite wave substantially continuous unidirectional voltages having magnitudes corresponding to the energy content of said pedestal portions. Y

7. A circuit for producing a voltage representative of the phase relation between impulses derived from an oscillator and impulses derived from a source of standardizing frequency comprising, a rectifier tube Aconnected to produce a series of impulses of twice the standardizing frequency, a vacuum tube coupled to 'said rectifier for. converting the impulses derived from said rectifier into a series of steep sidedimpulses, a relatively small capacity coupling condenser associated with said vacuum tube for Vimparting sloping crests to said steep sided impulses. a frequency divider connected tosaid oscillator for producing ai series of impulses having twice the standardizing frequency.

small/coupling condenser associated with said second vacuum tube for imparting saw-toothed i 'wave forms to said oscillator derived impulses, a

common output circuit for said vacuum tubes for combiningV said two series of impulses, whereby to produce a composite wave having pedestal portions, said pedestal portions having energy contents representative of the phase relation between said twoseries of impulses, andan integrating network for developing from said composite wave substantially continuous unidirectional voltages having magnitudes corresponding to the energy content of said pedestal portions.

8. A circuit for producing a voltage representa- Y tive of the phase relation between impulses derived from an oscillator and impulses derived from a source of standardizing frequency comprising, a full wave rectifier tube connected to said source of standardizing frequency and hava second vacuum tube coupled to said frequency divider, a relatively ing circuits to produce a series of impulses of twice the standardizing frequency, a vacuum tube coupled to said-rectifier and operated in the Spaoe current saturation region thereof for converting the impulses derived from said rectiiler into a series oi' steep sided impulses, a relatively small capacity coupling condenser associated with said vacuum tube for imparting sloping crests to said .steep sided impulses, a frequency divider connected to said oscillator for producing a series of impulses having twice the standardizing frequency. aI second vacuum tube coupled to said frequency divider, a relatively small coupling condenser associated with said second vacuum tube for imparting saw-toothed wave forms to said oscillator derived impulses. a common output circuit for said vacuum tubes for combining said two series of impulses, whereby to produce a composite wave having pedestal portions. said pedestal portions having widths representative of the phase relation between said two series of impulses, and an integrating network for developing from saidcomposite wave substantially continuous unidirectional voltages having magnitudes corresponding to the energy content of said pedestal portions.

9. A circuit for producing a voltage representative of the phase relation between impulses derived from an oscillator and impulses derived from a source of standardizing frequency comprising, a full wave rectifier tube connected to said source oi standardizing frequency and having circuits to produce a series of impulses of twice the standardizing frequency, first and second tandem connected vacuum tubes coupled to said rectifier and operated in'the space current saturation regions` thereof for converting the impulses derived from said rectifier into a series of steep sided impulses, a relatively small capacity coupling condenser associated with the second of said vacuum tubes for imparting sloping crests to said steep sided impulses. a frequency divider connected to said oscillator for producing a sexies of impulses having twice the standardizing frequency, a third vacuum tube coupled to said frequency divider. a relatively small coupling condenser associated with said third vacuum tube for imparting saw-toothed wave forms to said oscillator derived impulses, a common output circuit for-said second and third vacuum tubes for combining said two series of impulses, whereby to produce a composite wave having pedestal portions, said pedestal portions having peak amplitudes and widths representative of the phase relation between said two series of impulses, and an integrating network for developing from said composite wave substantially continuous unidirectional voltages having magnitudes corresponding to the energy ,content of said pedestal w means for combining said two series of impulses portions. v

10. In automatic frequency control apparatus' for a television timer, 'a "source of alternatingA current. an oscillator having an output to be maintained in harmonic relation to said alternating current source, means for converting said alternating current into a series of impulses having sloping crests, means for deriving from said oscillator a series of substantially saw-toothed impulses, each of said saw-toothed impulses having one edge of relatively great slope and the other edge of relatively small slope, means for combining said two series of impulses to form a ,composite wave having pedestal portions, said pedestal portions having widths representative of the phase relation between said two series of impulses, and means controlled by energy derived from said pedestal portions to maintain the out- 10 put frequency of said oscillator in said harmonic relation to said alternating current source.

11. In automatic frequency control apparatus for a television timer, a source of alternating current, an oscillator having lan output to be maintained in harmonic relation to said alternating current source, means for converting said alternating current into a series of steep sided impulses having sloping crests, means for deriving from said oscillator a series of substantially sawtoothed impulses having leading edges of relatively great slopes and trailing edges of relatively small slopes, means for combining said two series of impulses to form a composite wave having pedestal portions, said pedestal portions having peak amplitudes and widths representative of the phase relation between said two series of impulses, and means controlled by energy derived from said pedestal portions to maintain the output frequency of said oscillator in said harmonic relation to said alternating current source.

12. A ,circuit for producing a voltage representative of the phase relation between impulses derived from two periodic sources, means for deriving from one of said sources a first series of impulses, means for deriving from the other of said sources a second series of impulses having a substantially saw-toothed wave form, one edge of each oi' said saw-toothed impulses being of relatively great slope and the other ledge of relatively 40 small slope, means for combining said two sexies of impulses to produce a composite wave having pedestal portions of varying widths representative of the phase relation between said two series of impulses, and means for developing from said a composite wave substantially continuous unidirectional voltages having magnitudes corresponding to the energy content of said pedestal portions.

13. A circuit .for producing a voltage repre- '50 i sentative of the phase relation between impulses derived from an oscillator and impulses derived from a source of standardizing frequency comprising, means for deriving from said source of standardizing frequency a first series of impulses,

means for deriving from said oscillator a second series of impulses having a substantially sawtoothed wave form, one edge of each of said saw-l toothed impulses being of relatively great slope and the other edgel of relatively small slope,

to produce a composite wave having pedestal portions of varying widths representative of the i phase relation between said two series of im- 

